FIG. 1 is a block diagram of a conventional wireless direct-conversion receiver. The high-frequency signal, received by an antenna, is amplified by a low-noise amplifier (LNA) 1 and a programmable gain amplifier (PGA) 2. Then the signal is directly down-converted to a complex baseband signal by a quadrature mixer 3, receiving two orthogonal local oscillator (LO) signals LO-I and LO-Q, with the frequency of the fundamental wave of the signals LO-I and LO-Q equaling the mid-band frequency of the high-frequency signal. The in-phase signal I′, down-converted by LO-I, is fed in an in-phase baseband path of the receiver, whereas the quadrature-phase signal Q′, down-converted by LO-Q, is fed in a quadrature-phase baseband path of the receiver. Owing to various effects the two baseband signals I′ and Q′ at the outputs of the mixer 3, may be affected by an unknown and unpredictable DC-offset.
One reason for a DC-offset may be LO-leakage. Owing to a limited isolation, the LO-signal LO-I or LO-Q may inject in the high-frequency signal input of the mixer 3 or in the input of the LNA 1 or the PGA 2. The injected LO-signal is mixed with LO-signals LO-I and LO-Q, resulting in a DC component at the outputs of the mixer 3.
Yet another reason for a DC-offset may be self-mixing. In the case of self-mixing, the high-frequency signal at the input of the mixer 3 is injected to the LO-ports of the mixer 3. Thus, the high-frequency signal is self-mixed with an attenuated image of the high-frequency signal, resulting in a DC component at the outputs of the mixer 3.
The DC-offset is typically time variant, depending on the gain setting in the high-frequency part of the receiver. In communication systems with a plurality of possible transmitters as in WLAN-systems (wireless local area network) according to IEEE 802.11 and in ultrawideband (UWB) systems, in particular in multiband orthogonal frequency division multiplexing (MB-OFDM) systems, the offset at the output of the mixer 3 may change from burst to burst as the transmitter may change.
Additionally, in band hopping systems, e.g. in MB-OFDM systems or in Bluetooth systems, the DC-offset may vary from frequency band to frequency band as the LO-leakage effect is frequency dependent.
Each DC-offset, present at both outputs of the mixer 3, is further amplified by the cascaded PGAs 4a/b and 5a/b in the I- and the Q-path of the receiver and could lead to a signal clipping in the baseband section. Additionally, the offset reduces the dynamic range, available at the receiver chain and especially at the analog-to-digital converters (ADC), which are typically located at the output of the receiver in FIG. 1.
In addition to the offset generated by the mixer 3, the stages in front of the mixer 3, i.e. the LNA 1, the PGA 2 and the stages connected behind or after the mixer 3, i.e. the PGAs 4a/b and 5a/b and the LPFs (low-pass filters) 6a/b and 7a/b, may introduce further DC-offsets, e.g. by a parameter mismatch between transistors of a transistor pair. These additional DC-offsets may also vary in time, depending on the particular gain setting, temperature variations and power supply variations.
Conventional analog or digital DC-offset correction circuits typically employ a feedback-loop to correct the DC-offset. Conventionally, the signal in the receiver chain is coupled (via an ADC in a digital implementation) to a controller means, with the controller means generating an offset correction signal, which is fed back (via a DAC, i.e. a digital-to-analog converter, in a digital implementation) to the receiver chain.
In modern wireless communication standards based on frequency hopping, the band transition time is relatively short, e.g. roughly 10 ns for an MB-OFDM communication system. After this band transition time the band-specific DC-offset should be significantly eliminated in a short time interval (roughly 30 ns). Conventional DC-offset correction circuits are typically not capable of correcting the DC-offset in such a short time interval.